1. Field of the Invention
The invention relates generally to positioning systems and more particularly to a hybrid positioning system for locating and tracking an object using a combination of GPS, pseudoranges determined at the object from the global positioning system and radio pseudoranges determined for a radio signal transmitted from the object through one or more receptors to a base station.
2. Description of the Prior Art
There are many applications where it is necessary for a base station to know the position, identification and contents of a remote object. For example, asset management systems require periodic identification of the location and contents of remote containers in a warehouse and in transit. Cellular telephone service according to FCC regulations will soon require location of a cellular telephone that dials 911. Vehicle dispatch systems are required to track the locations of fleet vehicles. Applications as diverse as mining, agriculture, road paving, forestry, and construction require the precise locations of active machinery. The location accuracy required by such applications varies from several hundred meters for a container on a ship at sea to a few centimeters for active machinery.
The global positioning system (GPS) has now become the standard for location and navigation applications where a GPS receiver has clear view of a major portion of the sky. The GPS receiver uses GPS signals for determining ranges to GPS satellites, called pseudoranges because the internal clock in the GPS receiver is not synchronized to GPS time until the location of the GPS receiver is known. The location of the GPS receiver is calculated from the pseudoranges and ephemeris data for the locations-in-space of the GPS satellites. When location is needed at a base station but not at the GPS receiver, the GPS receiver transmits the GPS pseudoranges along with the satellite identifications and measurement times to the base station where current ephemeris data is stored. With differential GPS corrections, suitably equipped GPS receivers conventionally achieve location accuracy of a few meters to less than a meter. However, the GPS signal is relatively weak when it reaches the Earth from a GPS satellite and the additional attenuation of passing through foliage, into a building, around canyon or building walls, or the like may effectively block some or all the GPS signals. Furthermore, multipath of the GPS signal due to buildings or canyon walls and other objects may significantly reduce the accuracy of the location.
Normally, GPS signals from four GPS satellites are required for determining location. A location can be determined from fewer than four GPS satellites when other location information such as altitude, time, or map matching is available. However, more than four GPS satellites are often used for an overdetermined solution in order to improve accuracy. Existing GPS applications use pseudolites to augment the satellite constellation and thus improve availability of the GPS signal. Such pseudolites mimic the satellite transmissions by broadcasting pseudo GPS signals, but are fixed on the ground and transmit the location-determination information appropriate to the geographical location of the pseudolite. Signal reception is nearly guaranteed when the pseudolite is located nearby due to relatively higher signal strength of the received pseudo GPS signal. The pseudolites make use of PRN codes that have not been allocated for GPS satellites. In addition to the thirty-two PRN codes allocated for GPS satellites the United States government currently allocates about four codes for the use of pseudolites. For example, a pair of pseudolites at the end of an airport""s runway are conventionally used to enhance the position determination of a GPS navigational receiver in a landing aircraft. When the number of GPS satellites that can be received is fewer than the desired number due to impediments in the signal paths from the GPS satellites, the GPS signal is said to be partially blocked. It has been proposed that several pseudolites be used in a metropolitan area in order to improve GPS service in urban canyons and inside of buildings. Unfortunately, the use of GPS pseudolites has been limited due to their expense, the limited number of codes that have been allocated, and the possibility of jamming the satellite signals. The benefit of pseudolites within or near buildings or canyons is further limited by the effect of signal multipath on location accuracies.
Many electronic location determination systems are available or have been proposed that could conceivably augment the GPS system to provide electronic location information to a user equipped with a location determination receiver. Ground-based location determination systems, such as Loran, Omega, TACAN, Decca, U.S. Airforce Joint Tactical Information Distribution System (JTIDS Relnav), or U.S. Army Position Location and Reporting System (PLRS), use the intersection of hyperbolic lines or surfaces to provide location information. Unfortunately, none of these systems provide the location accuracy that is sometimes required. For example, LORAN-C provides a location accuracy that is typically within about 400 meters depending upon local conditions. A limitation of a LORAN-C location determination system is that not all locations in the northern hemisphere, and few or no locations in the southern hemisphere, are covered by LORAN-C. A second limitation of LORAN-C is that the location accuracy is insufficient for many applications. A third limitation of LORAN-C is that weather, local electronic signal interference, poor crossing angles, closely spaced time difference hyperbolas, and skywaves frequently cause the accuracy to be significantly worse than 400 meters.
Other ground-based location determination devices use systems that were developed primarily for communications, such as cellular telephone, FM broadcast, and AM broadcast. Some cellular telephone systems provide estimates of location, using comparison of signal strengths from three or more sources. FM broadcast systems having subcarrier signals can provide estimates of location by measuring the phases of the subcarrier signals. U.S. Pat. No. 5,173,710 by Kelley et al. discloses a system that allows determination of a location of a vehicle. In Kelley et al. FM subcarrier signals are received from three FM radio stations with known locations but unknown relative phases by signal processors at the vehicle and at a fixed station having a known location. The fixed station processor determines the relative phases of the signals transmitted by the three FM radio stations and transmits the relative phase information to the vehicle. The vehicle processor determines its location from the FM subcarrier signal phases and from the relative phase information it receives. Unfortunately, all of these systems are limited by their lack of accuracy, multipath reduction of accuracy, the region of operation, and/or the cost of the receivers or required infrastructure.
There is a need for a system for augmenting the global positioning system for providing high accurary at a relatively low cost where GPS signals are partially blocked.
It is therefore an objective of the present invention to provide a hybrid location system for locating a remote object using a combination of the global positioning system (GPS) and a radio pseudoranging system.
Briefly, in a preferred embodiment, a hybrid location system of the present invention includes a base station, one or more receptors at known locations having known signal transit times to the base station, and one or more location markers. The location markers are located with an object whose location is to be determined. Each of the location markers includes a marker GPS receiver and a radio transceiver. The marker GPS receiver includes a GPS pseudorange detector for determining GPS pseudoranges. The radio transceiver includes a radio transmitter for transmitting a radio signal including information for the GPS pseudoranges, the identification of the location marker, and typically the contents, characteristics, and/or identification of the object. Each of the receptors includes a radio receiver for receiving the radio signal and a communication interface for forwarding the information in the radio signal to the base station. The base station includes a radio pseudorange detector, a base GPS receiver, and a hybrid pseudorange processor. The radio pseudorange detector uses the time-of-arrival of the interface signal and the calculated transit time for determining a radio pseudorange from the location marker to the receptor. The base GPS receiver provides the locations-in-space for GPS satellites and preferably determines differential corrections for the GPS pseudorange. The hybrid pseudorange processor uses radio pseudorange calculations using the measured radio pseudoranges and the known locations of the receptors; GPS pseudorange calculations using the measured GPS pseudoranges, the locations-in-space of the GPS satellites, and preferably the differential corrections, or a combination of radio and GPS pseudorange calculations for four or more radio and/or GPS pseudoranges for determining the location of the object. Preferably, the system is time synchronized by transmitting the radio signal according to a locally generated GPS reference time used for determining the GPS pseudoranges and determining the radio pseudoranges with actual GPS-based time provided by the base GPS receiver. One or more of the location markers may be located with one or more of the receptors, respectively, for determining the locations of the receptors.
An advantage of a hybrid location system is that a location of an object can be determined when GPS signals are partially blocked by augmenting the GPS system with low cost location markers, receptors, and a base station of the present invention.
These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiments which are illustrated in the various figures.